feat(ui): add sport backgrounds to share cards, achievement filtering, and wizard validation

- Add ShareCardSportBackground with floating sport icons for share cards
- Share cards now show sport-specific backgrounds (single or multiple sports)
- Achievement collection share respects sport filter selection
- Add ability to share individual achievements from detail sheet
- Trip wizard ReviewStep highlights missing required fields in red
- Add FieldValidation model to TripWizardViewModel

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Trip/ViewModels/TripWizardViewModel.swift

@@ -77,6 +77,17 @@ final class TripWizardViewModel {
         hasSetRepeatCities
     }
 
+    /// Field validation for the review step - shows which fields are missing
+    var fieldValidation: FieldValidation {
+        FieldValidation(
+            sports: selectedSports.isEmpty ? .missing : .valid,
+            dates: hasSetDates ? .valid : .missing,
+            regions: selectedRegions.isEmpty ? .missing : .valid,
+            routePreference: hasSetRoutePreference ? .valid : .missing,
+            repeatCities: hasSetRepeatCities ? .valid : .missing
+        )
+    }
+
     // MARK: - Sport Availability
 
     func canSelectSport(_ sport: Sport) -> Bool {
@@ -120,3 +131,18 @@ final class TripWizardViewModel {
         mustStopLocations = []
     }
 }
+
+// MARK: - Field Validation
+
+struct FieldValidation {
+    enum Status {
+        case valid
+        case missing
+    }
+
+    let sports: Status
+    let dates: Status
+    let regions: Status
+    let routePreference: Status
+    let repeatCities: Status
+}